The sphygmomanometric class of automated blood pressure monitors employs an inflatable cuff to exert controlled counter-pressure on the vasculature of a patient. One large class of such monitors, exemplified by that described in U.S. Pat. Nos. 4,349,034 and 4,360,029, both to Maynard Ramsey, III and commonly assigned herewith, employs the oscillometric methodology. In accordance with the Ramsey patents, an inflatable cuff is suitably located on the limb of a patient and is pumped up to a predetermined pressure above the systolic pressure. The cuff pressure is then reduced in predetermined steps to a pressure below diastolic pressure, and at each step, pressure oscillations are monitored. On the other hand, the pressure may be reduced linearly or exponentially to a pressure below diastolic pressure before the pressure is exhausted to a near-zero pressure. The resultant signals typically consist of a DC voltage with a small superimposed variational component caused by arterial blood pressure pulsations (referred to herein as "heart cycles," "oscillation complexes" or just simply "oscillations"). After suitable filtering to reject the DC component and to provide amplification, peak pulse amplitudes (PPA) above a given baseline are measured and stored. As the decrementing of cuff pressure continues, the peak pulse amplitudes will normally increase from a lower level to a relative maximum, and thereafter will decrease. These amplitudes thus form an oscillometric blood pressure envelope for the patient. The cuff pressure at which the oscillations have a maximum value has been found to be representative of the mean arterial pressure ("MAP"). Systolic and diastolic pressures can be derived either as predetermined fractions of MAP, or by more sophisticated methods of direct processing of the oscillation complexes.
The step deflation technique as set forth in the Ramsey patents is the commercial standard of operation. A large percentage of clinically acceptable automated non-invasive blood pressure monitors ("NIBP") utilize the step deflation method. When in use, the blood pressure cuff is placed on the patient and the operator sets a time interval at which blood pressure measurements (i.e., cuff inflations) are to be made. The non-invasive blood pressure monitor automatically starts a blood pressure determination at the end of the set time interval. Typically, the user selects a short interval if the patient is unstable (since blood pressure may change to dangerous levels in a short time) and selects a longer interval as the patient becomes more stable.
In those situations where a short time interval is necessary, for example, in the so-called "stat mode," several cycles of blood pressure determinations are performed in rapid succession over a period of time such as five minutes. The stat mode of operation is utilized to rapidly track blood pressure, as when a patient is unstable and changes in the patent's blood pressure must be quickly detected and medical personnel alerted, if necessary. The clinician's need in stat mode is for as many readings of blood pressure and pulse rate as possible, as quickly as possible. This need must be balanced against some minimum time between cuff inflations (wait time) which is need to allow for circulation to the extremity distal from the cuff and to allow for venous blood to return from the extremity. In addition, any discomfort experienced by the patient while having cuff pressure on his or her limb for an extended period of time must be minimized.
Typically, these considerations are addressed in conventional systems by using a fixed interval between measurement cycles in the stat mode of operation. For example, the measurement cycle begins 3 seconds after the cuff pressure in the current cycle has fallen below 10 mm Hg. However, this approach does not account for the physiological minimum wait time between cuff inflation cycles which is necessary to allow venous blood flow to return from the distal end of the limb in order to minimize venous pooling. Thus, in the stat mode of operation, if the wait time is too short, the patient will experience pain and discomfort due to the reduced circulation. On the other hand, if the wait time is too long, dangerous changes in pressure may go undetected, which could lead to disastrous results if it is wrongly assumed that the patient's blood pressure is stable.
An example of a convention blood pressure measuring apparatus having a stat mode of operation may be found in U.S. Pat. No. 4,967,757, to Frankenreiter. The blood pressure measuring device of Frankenreiter determines a patient's blood pressure by inflating a cuff to a pressure P.sub.max above the systolic pressure to occlude an artery within the patent's limb. The cuff pressure is incrementally reduced to a pressure P.sub.min below the diastolic pressure. A time t.sub.next to begin a next blood pressure measurement is determined as a function of the time required to perform the present measurement cycle. In particular, Frankenreiter discloses a first time interval .DELTA.t.sub.pre in a measurement cycle that begins after the cuff pressure reaches a threshold pressure P.sub.thr, (e.g., MAP), during inflation. The time interval .DELTA.t.sub.pre ends when the cuff pressure reaches the threshold pressure P.sub.thr during the cuff deflation sequence. A second time interval .DELTA.t.sub.preII, which is equal to .DELTA.t.sub.pre, begins immediately after the cuff pressure reaches P.sub.thr. After time period .DELTA.t.sub.preII expires, the next cuff inflation cycle begins. According to Frankenreiter, the total time period of .DELTA.t.sub.pre and .DELTA.t.sub.preII provides for a dead time t.sub.dea between measurement cycles. However, Frankenreiter describes situations where the total time period of .DELTA.t.sub.pre and .DELTA.t.sub.preII fails to yield the proper wait time. For example, a predetermined minimum wait time .DELTA.t.sub.min is used if the cuff pressure is greater than P.sub.min upon the expiration of .DELTA.t.sub.preII. In addition, a predetermined maximum wait time period .DELTA.t.sub.max is used if the time period .DELTA.t.sub.pre should be unusually long. Thus, while under most circumstances the Frankenreiter system determines a wait time between measurement cycles, there are circumstances where operator-entered default time periods are used, which may not be a proper time period to allow for venous blood that has accumulated in a patient's extremity to return, nor properly provide for patient comfort and safety.
Another example of a blood pressure measurement apparatus is described in U.S. Pat. No. 4,889,133, to Nelson et al. The Nelson et al. apparatus raises the cuff pressure to a level above the systolic pressure and decreases the cuff pressure in a step-wise manner to an ending pressure which is less that the diastolic pressure. Pulsations due to heart contractions are detected at each of the decreasing cuff pressures. An impulse value, which is related to the detected pulsations, is determined from the area under the pressure waveform up to the maximum pulsation amplitude at each cuff pressure. The MAP is determined from the lowest cuff pressure having the largest waveform area (i.e., impulse area,). In addition, a prediction curve is initially generated from the first two impulse values and smoothed by subsequent impulse values in order to predict the next impulse value, reject invalid readings, and form a final smoothed curve. From the final curve, blood pressure parameters such as the systolic and diastolic pressure may be derived and displayed. While Nelson et al. discloses a technique whereby the area of the pressure waveform is utilized, the Nelson et al. technique fails to directly address the Stat mode of operation, nor does Nelson et al. address determining a waiting period between monitoring cycles.
Yet another example of a NIBP measurement apparatus is that of U.S. Pat. No. 5,606,977, to Ramsey, III et al., which is commonly assigned herewith. Ramsey discloses a blood pressure monitoring device that makes a complete determination of a patient's blood pressure at user-specified intervals. At the end of the user-specified interval, an inflatable cuff is pumped up to a pressure above the systolic pressure. The cuff is then incrementally deflated, and at each level, pressure fluctuations are monitored to form an oscillometric blood pressure envelope which is stored by the monitoring device. Between the user-specified intervals, the device is periodically inflated to a check pressure below the mean arterial pressure ("MAP"), and certain points on the patient's oscillometric envelope at the check pressure are compared to the stored oscillometric envelope derived from the previous complete blood pressure determination. If the signals from the check pressure determination differ by a predetermined amount from the stored signals, it is determined that the patient's blood pressure has changed significantly and that a complete blood pressure determination should be initiated immediately (i.e., before the expiration of a user-specified time interval). Otherwise, further check inflates are performed until the user-specified time period expires or it is otherwise determined that a complete inflate cycle should be conducted. According to Ramsey, inflations to the check pressure in this so-called "guard mode" of operation are conducted at a cycle time which is 1/50 that of the user-specified time period for a complete blood pressure determination, and are performed in addition to the complete determinations that occur at the end of the user-specified time period. While the check inflate sequence in the "guard mode" reduces the amount of trauma caused to the patient's limb as compared to a complete blood pressure determination, the "guard mode" sequence does not repeatedly collect data to establish a baseline as in the stat mode of operation. In any case, in the guard mode, the time between cycles is set by the operator and is not related to the time needed for venous return flow.
It is, accordingly, an object of the present invention to provide a system which automatically determines a minimum wait time between cuff inflations of blood pressure monitoring sequences. It is also an object of the invention to provide a system that adapts the wait time to minimize the level of patient discomfort by minimizing venous pooling. It is also an object of the present invention to provide a technique for monitoring the status of the patient's blood pressure which provides for faster blood pressure updates, thus increasing patient safety.